ISr6209-1982 Indian Standard ( Reaffirmed 2006 ) METHODS FOR PARTIAL DISCHARGE MEASUREMENTS ( First Revision ) First Reprint MAY 1996 UDC .: k 621.317.333.6.082-77 . 0 Copyright 1982 BUREAU MANAK OF INDIAN STANDARDS MARG BHAVAN, 9 BAHADUR SHAH ZAFAR NEW DELHI 110002 Gr7 Septcmbn 1982 I Indian Standard METHODS FOR PARTIAL DISCHARGE MEASUREMENTS ( High Voltage First Revision ) Sectional Committee, Representing Indian Institute of Science, Bangalore ETDC 19 Techniques Chairman DR B. I. GURURAJ Members DR G. R. NA~ABHUSHAN ( Alternate to Shri B. I. Gururaj ) SHRI A. K. BARMAN Calcutta Electric Supply Corporation Ltd, Calcutta SHRI S. MIJKHOPADHYAY ( Alternate ) DR S. C. BIIATIA Siemens India Ltd, Bombay SHRI V. T. D'SILVA ( Alternate ) Punjab State Electricity Board, Patiala SRRI A. K. CHOPRA SHRI P. S. SATNAX ( Alternate ) Jyoti Ltd, Vadodara SHRI V. B. DESAI DR P. SATYANARAYANA ( Alternate ) DIRECTOR Central Power Research Institute, Bangalore SENIOR DEPUTY DIRECTOR ( Alternate ) DIRECTOR ( EHV ) Central Electricity Authority, New Delhi DIRECTORI SUB-STATION , 1( .-Alternate 1 JOINT DIRE&R STANDARDS Research, Designs & Standards Organization, ( TI-1 ) Ministry of Railways, Lucknow DY DIRECTOR STANDARDS (E) Cs ( Alternote ) DR M. V. Josur Electric Research & Development Association, Bombay SERI P. K. JOSHI ( Aftmate ) SHRI V. N. MANOHAR Tata Hydro-Electric Power Supply Co Ltd, Bombay SHRI G. K. VENKATRAO ( Alfernare ) MISS MARY MATHEW Tamil Nadu Electricity Board, Madras SHKI S. KRISHNARAJ ( Alternate ) SHRI S. K. MUKRERJEE National Test House, Calcutta SHRI R. N. M~EEERJEE ( Alternate ) { Confinued on page 2 ) .-BUREAU @ cowripht OF INDIAN 1982 STANDARDS ! This publication is protected under the Indiun Co@ighr Act ( XIV of 1957 ) and reproduction in whole or in part by any means except with written permission of the publisher shall be deemed to be an infringement of copyright under the said Act. I ISr6269-1982 ( Conlinued~ompuge 1 ) Members SHRI V. H. NAVKAL R+mnting Bombay Electric Bombay Supply & Transport Undertaking, SWRI M. L. DONQRE ( Alternate ) Bharat Heavy Electricals Ltd, New Delhi SERI T. PHILIP SHRI B. N. GHOSR ( Ahrnotc ) SRRI K. NATARAJAN( Altemafs) Hindustan Brown Boveri Ltd, Bombay SERI S. R. POTNIS SRRI V. S. MANX ( Alternate ) Karnataka Electricity Board, Bangalore SHRI K. S. SIVAPRAKASAM SFIRI H. M. S. LIN~AIR ( AIternYc ) U. P. Government Pottery Development SHRI SURENDRA SINCJH Khurja SHRI KOJKAL SINQR ( AIternatc ) W. S. Insulators of India Ltd, Madras SERI K. TEIRUVENEADATHAN SRRI S. RAJAN ( Alternate ) Crompton Greaves Ltd, Bombay SHRI C. R. VARIER DR G. PARTHASARATHY( Af:ernntc) Director General, IS1 ( Ex-o&i0 Member ) SlIR1S.P. SACliDEV, Director ( Elec tech ) Centre, SARI K. GANPXIE Assistant Director ( Elec tech ), IS1 Indian Standard METHODS FOR PARTIAL DISCHARGE MEASUREMENTS ( First Revision ) 0. FOREWORD 0.1 This Indian Standard ( First Revision ) was adopted by the Indian Standards,Institution on 28 April 1982, after the draft finalized by the High Voltage Techniques Sectional Committee had been approved by the Electrotechnical Division Council. 0.2 This standard was originally published m 1971. This revision has been undertaken with a view to align with the latest developments at the international level. The important features of the revision are as follows: a) Includes information regarding of measuring instruments. frequency response characteristics b) Additional non-electrical method, that is, method of dissolved gases in oil, for detection of partial discharges has been included. c) Additional methods of calibration have been included. disturbance level has main d) Additional methods regardin, 0 reductionof been included. 0.3 Measurements purposes: of partial discharges are made for the following a) To verify that the test object does not exhibit partial discharges greater than a specified magnitude, at a specified vo!tage; b) To determine the voltage amplitude at which partial discharges of a specified low magnitude commence with increasing voltage and cease with decreasing voltage; and c) To determine the magnitude at a specified voltage. 3 of the specified discharge quantity I6:626!3-1962 0.4 The discharges which are considered in this standard are localized electrical discharges in insulating media, restricted to only a part of the dielectric under test and only partially bridging the insulation between Each individual discharge causes a single pulse of current conductors. in the dielectric and in the external circuit. Even though they iuvolve only small amounts of energy, partial discharges may lead to progressive deterioration of the dielectric properties of insulating materials. The definitions and evaluation of such deterioration, however, are beyond the scope of this standard. Partial discharges may occur in cavities of solid insulation, in gas bubbles, in liquid insulation or between layers of insulation with different They may also occur at sharp edges or points dielectric characteristics. of metallic surfaces. 0.5 Partial discharge measurements in apparatus having windings, such as transformers, generators, motors are complicated by the attenuation, resonance and travelling wave phenomena. Special requirements for tests on these objects are only briefly dealt with. 0.6 The objects of this standard are: a) to define the terms used; b) to define the relevant quantities to be measured; c) to describe test and measuring circuits which may be used; d) to recommend some types of measurement and instrumentation suitable for particular applications; e) to recommed methods for calibration; and f) to describe test procedures. 0.7 In the preparation of this standard, assistance has been derived from IEC Document 42 ( Secretariat ) 33 `Partial Discharge Measurements', issued by the International Electrotechnical Commission. 0.8 For the purpose of deciding whether a particular requirement of this standard is complied with, the final value, observed or calculated, expressing the result of a test or analysis shall be rounded off in accordance with IS : 2-1960*. The number of significant places retained in the rounded off value should be the same as that of the specified value in this standard. 1. SCOPE 1.1 This standard applies to the measurement of partial discharges during tests with alternating voltage, but general terms, definitions and require*Rules for rounding off numerical values ( rcuiscd ). 4 IS:6269-1982 ments are usually also applicable for measurement of partial discharges Some special characteristics of partial during tests with direct voltage. discharge measurements under direct voltage are given in a separate clause and necessary references are made throughout the text. The standard is intended principally as a general guide for drafting specifications for specific apparatus. 1.2 Even though they involve only small amouts of energy, partial discharges may lead to progressive deterioration of the dielectric properties of insulating materials; the definition and evaluation of such deterioration, however, are beyond the scope of this standard. 1.3 The standard deals mainly with electrical measurements of partial discharges, but some reference is also made to non-electrical methods. 2. TERMINOLOGY 2.1 Partial Discharge - A partial discharge, within the terms of this standard, is an electric discharge that only partially bridges the insulation Such discharges may, or may not, occur adjacent between conductors. to a conductor. NOTE `corona'. Partial discharges in gases around a conductor are sometimes referred to as This term should not be applied to other forms of partial discharges. The general term `ionization' should particular case of partial discharges. 2.2 Quantities not be used to denote the Related to Partial Discharges 2.2.1 General - Partial discharges occurring in any test object under given conditions may be characterised by different measurable quantities such as charge, repetition rate, etc. Quantitative results of measurements For are expressed in terms of one or more of the specified quantities. tests with direct voltage, see 8. 2.2.2 Apparent Charge, q - The apparent charge, q of a partial discharge is the absolute value of that charge which, if injected instantaneously between the terminals of the test object, would momentarily change the voltage between its terminals by the same amout as the partial discharge itself. The apparent charge is expressed in picocoulombs. NOTE 1 -The apparent charge is not equal to the amount of charge involved at the site of the discharge and which cannot be directly measured. locally NOTE 2 - In practice, the waveform of the voltage appearing across the terminals of the test object due to the partial discharge itself may be different from that produckd by the calibrating pulse. The apparent charge is considered to be that charge which, if injected between the terminals of the test object, will give the same reading on the measuring instrument as the partial discharge itself. Special cases are those in which the teat objects include travelling wave or attenuation phenomena ( SCI Appendix C ) . 5 IS:6209-1982 2.2.3 Repetition Rate, n - is the average number over a selected time. The partial discharge pulse repetition rate n of partial discharge pulses per second measured NOTE - In practice only pulses above a specified magnitude or within a specified range of magnitudes, may be considered. The results are sometimes expressed as cumulative frequency distribution curves of partial discharge magnitudes. Quantities - For 2.2.4 Integrated quantities which are characterized by are in use. This time interval is often of one cycle of an alternating voltage Examples of such quantities are: particular purposes integrated summations over a time interval T long compared with the duration applied to the test object. a) the average discharge current, I. charges during This is the sum of the absolute values of the apparent a certain time interval divided by this time interval. b) the quadratic rate, D. This is the sum of the squares of the apparent charges, during a certain time interval divided by this time interval. c) the discharge power, P. of the test object due This is the average power fed into the terminals to partial discharges. For particulars, see Appendix B. 2.3 Specified Partial Discharge Magnitude - The specified partial discharge magnitude is the vallle of the partial discharge quantity stated in standards or specifications for the given test object at a specified voltage. 2.4 Voltages Related to Partial Discharges - Voltage values during partial discharge tests are given by their peak values divided by The following voltages are of 42in the case of alternating voltages. For tests with direct voltages, see 8. particular interest. 2.4.1 Partial Discharge Inception Voltage, iY1 - The partial discharge inception voltage, Ul, is the lowest voltage at which partial discharges are observed in the test arrangement, when the voltage applied to the object is gradually increased from a lower value at which no such discharges are observed. In a practical case the inception voltage Vi is the lowest voltage at which the partial discharge magnitude becomes equal to or exceeds a specified low value. 6 ISx6209-Ido2 24.2 Partial Discharge Extinction Voftage, U. - The partial discharge extinction voltage Ue is the lowest voltage at which partial discharges are observed in the test arrangement, when the voltage applied to the object is gradually decreased from a higher value at which such discharges are observed. In a practical case, the extinction voltage Ue is the lowest voltage at which the partial discharge magnitude becomes equal to or less than a specified low value. 2.4.3 Partial Discharge Test Voltage - The partial discharge test voltage is a specified voltage, applied in a specified test procedure, during which the test object should not exhibit partial discharges exceeding a specified magnitude. 3. TEST CIRCUITS AND MEASURING INSTRUMENTS 3. I General Requirements 3.1.1 In this clause, various types of test circuits and instruments for Whatever the measurement of partial discharges are briefly described. type of test circuit and measuring instrument is used, they should be calibrated as specified in 4 and should meet the requirements specified by the relevant standard ( see 6 and 7 ). The relevant standard shall specify the particular quantity or quantities to be measured. Any imtrument measuring this quantity or quantities is in general considered acceptable. The standard may also recommend `a particular test circuit to be used. If not otherwise specified by the relevant standard, any of the test circuits mentioned in 3.2 and any of the instruments mentioned in 3.4 are acceptable. For tests with direct voltages, see 8. 3.1;2 Non-electrical methods of partial discharge detection are not recommended for quantitative measurements, but they are useful for Some information is special purposes, for exampIe, discharge location. therefore given in 3.3. 3.2 Test Circuits 3.2.1 Most circuits in use for partial discharge measurements can be derived from one or other of the three basic circkts, which are shown in Fig. la, lb and lc, some variations of these circuits are shown .in Fig. 2 and 3. Each of these circuits consists mainly of: a) a test object, which in many cases can be regarded C,, ( see however, Appendix C ); b) a coupling capacitor, Ck or a second test object %I; 7 as a capacitor, I8 t 6299 - 1982 c) a measuring circuit consisting of the measuring impedance .$,, ( and sometimes a second impedance & ), the connecting lead and the measuring instrument; d) sometimes an impedance or filter, 5, to prevent discharge pulses from being by-passed .through the high-voltage supply and to reduce interference from the source. 3.2.2 The particular characteristics ments are considered in Appendix A. of the different circuit arrange- IA 0 InstrumentConnected inSeries with a Coupling Capacitoa 1 I UN I -4 I6 lustrument Connected in Series with the Test Object I r IC Balanced Circuit Arrangements FIG. 1 BASIC PARTIAL DISCHARGE TEST CIRCUITS 8 ISt62693.3 Meamring Circuits 1682 3.3.1 Partial discharges in the test object cause charge transfer in the test circuit giving rise to current pulses through the measuring impedance. This impedance, in combination with the test object and coupling capacitor determines the duration and shape of the measured voltage pulses. These pulses are further shaped and amplified in order to supply to a measuring instrument a value proportional to the apparent charge quantity. 3.3.2 Depending on the frequency range of the measyrement, the measuring circuits can be classified into two groups: wide-band and The wide-band circuit exhibits a pulse response which narrow-band. permits the determination of amplitude and polarity of individual discharge pulses in addition to the discrimination between consecutive pulses. The narrow-band circuit normally permits only the determinaIn general, a widetion of the amplitude of the measured discharges. band circuit is more susceptible to external interference. The wide or narrow-band characteristics of the measuring circuit are normally determined by the instrument and the measuring impedance. 3.3.2.1 Measuring circuit characteristics - Characteristics of the measuing circuits are determined by the following parameters: a) Lower and ugper cut-offfrequencies, fi andfi - The lower and upper cut-off frequencies f, atid fa are the frequencies at which the response to a constant sinusoidal input voltage has fallen by a fixed amount, usually 3 dB, from the constant value in the case of wide-band circuits and 6dB from the peak value in the case of narrow-band circuits. b) 4 the responses show a resonance Resonance frequency, fo --`When peak ( narrow-band circuits or instruments ) the corresponding frequency is called the resonance frequency f . Bandwidth, A f - For both narrow-band ments, the bandwidth is defined by: A f and wide-band instru- = fi - fi For wide-band responses f, usually is of the same order to magnitude as fs whereas it is substantially smaller than fo for narrow-band responses. d) Pulse resolution time - The pulse resolution is the shortest time interval `,between two consecutive pulses which results in an amplitude error of not more than 10 percent due to superposition caused by the overlapping of the pulses. 9 The pulse resolution time is inversely' proportional bandwidth of the measuring circuit. to the 4 Sculsfatty of tke measuring circuit, kc - The scale factor kc, is the factor by which the reading of the instrument shall be multiplied to obtain the magnitude of the measured partial discharge . The scale factor kc is not the same as the scale factor ?rgFt% instrument alone. See 3.4.1 and 4.2. 3.3.22 Measuring impedance - The measuring impedance usually acts as a four terminal impedance with a frequency response chosen to prevent the test supply frequency from reaching the instrument. This may be achieved in the case of a resistive impedance by connecting an inductor in parallel with the resistor, or, by connecting a capacitor in series between the measuring resistor and the measuring cable. The measuring impedance may consist of a resistor, a resistor in parallel with a capacitor, a tuned circuit or a more complex filter design. For narrow-band measuring circuits, tuning of the measuring impedance to the measuring frequency of a instrument is often used. 3.3.3 Coupling Ca@citor -The coupling capacitor shall be of a low inductance design and its resonant frequency shall be not less than 3fp ' In addition, the coupling capacitor partial discharges at the test voltage. shall not exhibit any significant AlTERNATIVE POSITION FOR fm Fro., 2 3.4 Measuring TEST CIRCUIT FOR MEASUREMENT AT A TAPPING OF A BUSHING Instruments 3.4.0 Instruments available fbr partial discharge measurements may be classified in various ways. In 3.4.1 to 3.4.7, their main requirements 10 are summarized jn accordance ( defined in 2.3 and 2.4 ). with the quantities to be measured FIG. 3 TEST CIRCUIT FOR MEASURINGSELF-EXCITED TEST OBJECTS measurjng be used as discharges disturban.. Whatever other form of indication may be given by the instrument it is recommended that an osciloscope should also this assists in distinguishing between different types of partial and between the discharges to be measured and extraneous ces. See 8 for the case of tests with direct voltages. 3.4.1 Scale Factor of the Measuring Instrument, ki - The scale factor ki is the factor by which the reading of the instrument shall be multiplied in order to obtain the magnitude of the discharge quantity injected into the instrument during its calibration. 3.4.2 Instruments for the Measurement of Apparent Charge, q - The current pulses due to partial discharges produce a signal at the terminals of the measuring impedance. For short pulses the signal is a voltage pulse whose peak value is proportional to the apparent charge (q) of the test object ( see Notes 1 and 2 below ). The magnitude of the apparent charge which is measured during an actual test is generally understood to be that associated with the largest repeatedly occurring pulse. The individual pulses may be displayed on a cathode-ray oscil!oscope and the magnitude of the apparent charge can be determined by calibration. The pulses may be displayed on a linear time-base which is triggered, for example, by the discharge pulse or by of the test voltage. It may also be convenient to display .the pulses on an elliptical time-base which rotates synchronously with the test voltage frequency. The magnitude q of the largest discharge pulses can also be measured by a suitable peak meter. 11 l I IS:6289 -1982 The resolution time of the instrument is acceptable if no error in amplitude measurement occurs due to overlapping of pulses when these are at least 100 ps apart. Resolution times much shorter than this are desirable, however, and can be obtained with available instruments. Errors may also occur due to the time constants of the peak meter if the pulse repetition rate is low. NOTE 1 - Due to the nature of the partial discharge, or to capacitance elements in the measurement circuit, the current pulses might be lengthened. Then, for the longer pulses, the apparent charge at the test object is proportional to the intargtal of the voltage pulse. NOTE 2 -The distribution of the repetition rates which are lrelated pulse magnitudes can be determined using pulse counters. to different i 3.4.3 Instruments for the Measurement of Pulse Repetition Rate, n - Any kind of pulse counter or rate meter indicating either the total number of pulses in a given time or the average number per second for all amplitudes measured or for given amplitude ranges can be used for measurement of the repetition rate n, provided that the resolution time is sufficiently short. Usually, such counting instruments incorporate magnitude discriminators which suppress pulses below an adjustable predetermined magnitude. Some care is needed to avoid obtaining more than one count per pulse if the pulses reaching the counter are oscillatory. 3.4.4 Instrument for the Measurement of Average Discharge Current, I - In an instrument which measures the average value of the principle, discharge current pulses after linear amplification and rectification will indicate, when suitably calibrated, the average discharge current, I, as Precautions are necessary to avoid undetected errors defined in 2.2.4. either due to amplifier overloading at low discharge repetition rates, n, or to overlapping of oscillatory pulses when n is large. 3.4.5 Instrument for Measurement of Qpardratic Rate, D - An instrument which measures the mean square value of the discharge magnitudes per second will indicate the quadratic rate, D, as defined in 2.2.4. The measurement may be made by paqsing the amplified pulses through a rectifier giving square law response and deriving the resulting mean dc component; or alternatively, it may be made by passing the pulses from a linear amplifier into the thermal detector. The overload characteristics of the instrument require special consideration. 3.4.6 Arrangemeni for the M?asur?ment of Dkharge Power, P - Different types of test circuits and instruments are possible for the measurement of discharge power. They are usually based on the measurement of the area of -an oscilloscope display or on more sophisticated techniques. The calibration of such test circuits and instruments relies on the 12 IS : 6209 - 1982 determination charge. of the scale factor for applied voltage and apparent 3.4.f Use of Radio Interference Meters for the Measurement of Partial interference meters are frequency selective voltmeters. Discharges -Radio The instruments are primarily intended for measuring interference caused to reception of broadcast radio signals. Because of their special characteristics, radio interference meters do not directly indicate any of the partial discharge quantities defined in this standard ( 2.3 and 2.4 ) but they give a general indication of discharge magnitude when used on the quasi-peak setting and calibrated according to 4.3. See also Appendix D. The reading is particularly sensitive to the repetition rate of the It may be used provided that the pulse repetition rate dischage pulses. is greater than 50 per second. 3.5 Non-electrical Method of Detection 3.5.0 Non-electrical methods of partial discharge detection include acoustical and optical methods and also, where practicable, the subsequent observation of the effects of any discharges on the test object. In general these methods are not suitable for quantitative measurements of partial discharges and are essentially used to locate the discharges. 3.5.1 Acoustic Detection - Aural observations made in a room with low noise level may be used as a means of detecting partial discharges. Non-subjective accoustical measurements, usually made with microphones or other transducers and oscillscopes, have also been found useful, Directionally selective microparticularly for locating the discharges. phones with high sensitivity above the audible frequency range are useful for locating corona discharges in air. Transducers in combination with oscilloscopes may al30 be used for locating discharges in oil-immersed equipment, such as transformers; they may be either inside or outside the oil tank. 3.5.2 Visual or Optical Detection - VAual observations are carried out in a darkened room, after the eyes have become adapted to the dark and Alternatively, if necessary with the aid of binoculars of large aperture. a photographic record can be made, but fairly long exposure times are usually necessary. For special pupojes, photo-multipliers or image intensifiers are sometimes used. 3.5.3 Observations of Trackiq - Tracking by discharges may give useful information 13 mark; which have been left on the location and extent of IS : 6209 - 1982 discharges when subsequent inspection is possible. assisted by the use of ultra-violet !ight. Observation may be 3.5.4 Dissolved Gases in Oil - The presence of partial discharges in oil insulated apparatus may be detected in rome cases by the analysis of the gases dissolved in the oil. This is mually a long luraticn phenomenon and the measurement is not associated with normal dielectric tests. 4. CALIBRATION 4.1 General 4.1.0 Calibration involves two separate procedures; one is complete determination of the characteristics of the measuring instrument itself including a detailed calibration and should be performed after major repairs or at least once per year, the other is a routine calibration of the instrument in the complete test circuit and should be performed before every test or, if many identical test objects are being tested then it may be performed at suitable intervals to be determined by the user. The latter calibration should include a verification that the instrument, as used in the test circuit, will be able to measure the discharge level which has been specified by the relevant standard ( see 5.1). Some methods for calibration are described in 4.2 and 4.3; other methods may be ured if their applicability is demonstrated. 4.2 Determination of Instrument Characteristics I 4.2;0 The determination of the characteristics and the calibration of the measuring instrument should be made on all its ranges of measurement, in the conditions given by manufacturer specifications or by applicable standards. The measuring inpedance 5 m and any connecting cables should be included in the calibration of the instrument. The following characteristics should be determined: a) Variation of scale factor ,ki to pulses of different magnitude at low repetition rate ( about 100 per second ); b) Pulse resolution time, by applying at increasing repetition rate; c) Lower and upper cut-off frequencies, pulses of constant magnitude fi and fi; and d) Stability and accuracy of the calibrating devices. The characteristics may be considered acceptable if their values have not changes by more than some percent in one year. In this case the calibration is usually not necessary at shorter intervals. 14 `m:6209-1!m The eiror limits allowed for partial discharge measuring instruments are usually larger than in other measurements. Specific figures are under' consideration. 4.21 Calibration of Instruments Measuring Apparent Charge, q - Calibration of an instrument for the measurement of the apparent charge q of single partial discharges is carried out by passing short current pulses of any convenient but known charge magnitude, qo, through the instrument (or through the measuring impedance & ). Such pulses may be produced by means of a generator giving rectangular step boltages of amplitude Uo, in series with small known capacitance Co. Under these conditions, the calibration pulse is equivalent to a discharge of magnitude: 40 = uoco In practice, it may nqt be possible and even though other waveforms decay times may inject essentially the tion circuit responses will be different corresponding current pulses. to produce ideal step voltage pulses having slower rise times and finite same amount of charge, the detecdue to the different durations of the A calibration puls having a rise time of not more than O.Os& 111 a decay time in the range of 100 rS to some 1 000 rS will usually be suitable. As a source of calibration pulses with short fronts, small battery-operated pulse generators are in common use, employing either transistors or relays with mercury wetted contacts. When the main parameters ( Uo, Co ) of the pulse generator cannot be separately checked, then a functional check shall be made by comparison with an arrangement comprising a step voltage generator in series with a known capacitance. Precautions should be taken to ensure that the measurement of this capacitance is not disturbed by the presence of stray capacitances. 4.2.2 Calibration of Instruments M:zs:rring Zntegrzted Qxantitk - The use of a generator similar to that described in 4.2.1 giving pulses of known charge and repetition rate is applicab'e for th" calibration of instruments A calibra. measuring the average discharge current or qua Iratic rate. tion procedure is given in Appendix B. 4.3 Calibration Arrangement of the Instrament in the Complete Test 4.3.1 The calibration of the instrum,Tnt in the complete test arrangement is made to determine the scale factor kc with the test object connected. This factor is affzcte1 by the circuit characteristics, generally the seasitivity increases with inerealed value of coupling capacitor C,. The calibration should be repeated for each new tejt object, except in 15 cases where tests are made on a series of similar test objects having capacitance values within f 10 percent of the mean value. This calibration need only be made at one or a few values of the measured quantity. This calibration may be used to check the minimum discharge This minimum quantity is affected magnitude which may be measured. by the disturbance level and by the circuit characteristics ( SCG 7 ). 40 4 CONNECTIONS FOR THE CALIBRATION OF THE COMPLETE TEST ARRANGEMENT 4.3.2 Calibration of instruments measuring q, in the complete test arrangement should be made by injecting short current pulses into the In the case of terminals of the test object, as shown in Fig. 4a and 4b. the test circuit shown in Fig. 4b, it is important to note that if the calibration pulses were applied between the high-voltage terminal and ground, errors are likely to be introduced. 16 The calibration pulses are obtained in the same manner and should meet the same requirements as given in 4.2.1. In addition to these requirements, the value Co of the calibrating capacitor should not be larger than about 0 1 X ( Ca + ck ). The calibration pulse is then equivalent to a discharge magnitude: qo - Ua CO. In the case of tall test objects several metres in height the injection capacitor Co should be located close to the high voltag? terminal of the test objeci. In the same case, errors will also be cause.4 by any stray capacitances from the junction point of Co and the step voltage generator to the high voltage terminal unles.3 these are negligible in comparison with that of Co itself. 4.3.3 Despite the fact that a limitation of the bandwidth by the circuit itself is taken into account by the calibration, it is desirab!e that this limitation be avoided. Consequently;the resonance frequency f-, of a narrow band detector should satisfy: fo < O-3 fn which may be checked where by calculation c _ G G a ca-I- Ck a ( It1 + 1 + hz L = = 10-s H/m ): h, and hg are heights of test object and coupling capacitor and 1 is length of conductor between them. NOTE 1 - If the calculation shows resonance near the measuring frequency then the frequency response of the circuit has to be measured. When varying the measuring frequency over the range of fO f A J no considerable change of the scale factor kc should be observed. NOTE2 - In the case of test objects with distributed parameters which cannot be represented by circuits with lumped elements ( such as cables ) special calibrations techniques may be necessary. See Appendix C. 5. TESTS 5.1 General Requirements 5.1.1 In order to obtain reproducible results in partial discharge tests, The following clauses careful control of all relevant factors is necessary. give the requirements applying to the test object itself and to the tert voltage. Additional requirements, for special test conditions and methods 17 IS:6209-1982 of test, may be specified by the-relevant Iddian standard. This standard should(also specify the quantity to be measured and the minkurn measurRefertnce :hould be nade to 7.4 able discharge magnitude required. for information on practical limits of minircum rrearurable magnitude. For the case of tests with direct voltages, ICC8. 5.2 Conditioning ing procedure of the Teat Object 5.2.0 Befofole being tested, a test object should undergo the condition; specified by the relevant standard. 5.21 If not otherwise specified, the surface of the insulators should be clean and dry since moisture or contamination on insulating surfaces may cause partial discharges. In addition, the testobject should be at ambient temperature during the test. Mechnical, thermal and electrical stresiing just before the test may affect the result of partial discharge a rest interval after previous stressing tests. To ensure reproducibility, may be required before making partial discharge tests. 5.3 Requirements for the Test Voltage 5.3.1 For partial discharge tests with alternating voltages, the test voltages and the rate of rise shall comply with the requirements of IS : 2071 ( Part II )-I974 * if not otherwise specified. 5.4 Choice of Test Procedure 5.4.9 The specification of procedures to be used .for particular types of test and of test object is the responsibility of the relevant Indian standard. They include any preliminary conditioning process, the test voltage levels and frequency, sequence and durations of voltage application, and the relationship of partial discharge measurement tests to other dielectric tests. To assist in preparing such test specifications, three examples procedures for alternating voltages are given in 5.4.1 and 5.4.2. of test 5.4.1 Determination of the Partial Dischcrge Inception and Extinction Voltages A voltage well below the expected inception value is applied to the test object and gradually increased until discharges exceed a specified low The test voltage at this specified magnitude is the partial magnitude. discharge inception voltage. The voltage is then inrreared by about IO percent and thereafter reduced to a value at which the discharges cease The test voltage at or become less than the same specified magnitude. this discharge limit is the partial dkcharge extinction voltage. Note that with some insulation systems, the extinction voltage may be influenced by *Methods of high voltage testing: Part II Test procedures ( jir~t revision ). 18 IS:6209-1982 the time during which the voltage is mantained above the inception voltage. In the case of repeated measurements of the inception and extinction voltages, both voltages may be affected. Under no circuktances, however, should the voltage applied exceed the rated withstand voltage applicable to the apparatus under test. Note that, in the case of high voltage apparatus there is some danger of damage from repeated voltage applications approaching the rated withstand voltage. 5.4.2 Determination of the Partial Discharge Voltage 4 Measurement without prestressing The partial discharge magnitude. in terms of the specified quantity is measured at a specified voltage, which may be well .above the expected partial discharge inception voltage. The voltage is gradually increased from a low value to the specified The partial value and maintained there for the specified time. discharge magnitude is measured at the end of this time, and thereafter the voltage is decreased and switched off. Sometimes the magnitude of the discharges is also measured while the voltage is being increased or reduced or throughout the entire test period. b) Measurement with prestressing In an alternative procedure the test is made by raising the test voltage from a value below the specified partial discharge test voltage up to a specified voltage exceeding this voltage. The voltage is then maintained for the specified time and, thereafter, gradually reduced to the value of the partial discharge test voltage. At this voltage level, the voltage is maintained for a specified time and at the end of this time the partial discharge magnitude is measured in a given time interval. 5.5 Measurements on Cables and on Test Objects with Windings Magnitude at a Specijed Test . 5.5.1 Some guidance for the measurement of partial discharges on cables and in test objects with windings will be found in Appendix C. 6. MEASURING ACCURACY AND SENSITIVITY 6.1 Partial discharges are usually phenomena which are greatly affected by several factors and therefore are of relatively low reproducibility. Also, the measurements of partial discharges usually present larger IS : 6269 - 1982 errors than other measurements during high voltage tests. This should be taken into consideration when specifying partial discharge acceptance levels. The measurements are also affected by the background noise which should be low enough to permit a sufficiently accurate measurement of the partial discharge ( normally less than 50 percent of the specified permissible partial discharge magnitude ). 6.2 Pulses which are known to be caused by external disturbances maybe disregarded. Wh en low ( < 10 pC ) partial discharge magnitudes are specified for equipment acceptance tests, a background noise up to 100 percent of the specified value may be accepted. NOTE - The minimum partial discharge magnitude which may be measured in a particular test is in general limited by disturbances. However, where these are effectively eliminated by suitable screentng or by using a balanced test circuit, the limits are usually determined by the internal noise level of the instrument itself and bythe values of the test circuit parameters, especially C,,, Ck, &,, and any capaciIn general, the minimum measurable magnitude tance Cm in parallel with &,. increases with increase in the values of C,,, C,, I/&,, and the ratio CsjCk. The use of a matching transformer may increase the signal to noise ratio of the measurements for cases where the test object capacitance is very small or very large. 7. DISTURBANCES 7.1 Sources of Disturbances 7.1.1 Interference with the indication of partial discharge measuring instruments may be caused by disturbances which fall into two categories: Disturbances a> They may which occur even if the test circuit is not energized. be caused for example by switching operations in other circuits, commutating machines, high-voltage tests in the vicinity, radio transmissions, etc, including inherent noise of the measuring instrument itself. They may also occur when the power supply is connected but at zero voltage. b) Disturbances which only occur when the circuit is energized but which do not occur in the test object. These disturbances usually increase increasing voltage. with They may include for example, partial discharges in the testing transformer, on the high-voltage conductors, in bushings ( if not part of the test object ), or disturbances caused by sparking of imperfectly earthed objects in the vicinity. They may also be caused by imperfect connections in the area of the high voltage, for example, by spark discharges between screens and other high-voltage conductors, connected with the screen only for testing purposes. Disturbances may also be caused by higher harmonics of the test voltage within the bandwidth of the measuring instrument. Partial discharges or sparking contacts in the low voltage supply may also cause distur20 ~Is,62O!B=l!MJ$ bances if transferred through the test transformer connections to the measuring circuit. For the case of disturbances 7.2 Detecting Disturbances or through other with direct voltages, see 8. 7.2.1 The voltage-independent sources can be detected by a reading on the instrument when the test circuit is not energized. The value It is incorread on the instrument is a measure of these disturbances. rect to subtract this value from the measured partial discharge magnitude. The voltage dependent sources of disturbances can be detected in the The test object is either removed or replaced by an following manner. equivalent capacitor having no significant partial discharges. The circuit The circuit should should be recalibrated by the procedure given in 4.3. If the disturbance level now be energized up to the full test voltage. exceeds 50 percent of the maximum permissible discharge level of the test object, then measured should be introduced to reduce the disturbances. One or more of the methods described in 7.3 may be used to reduce the disturbances. The use of au oscilloscope as an indicating instrument helps the observer to distinquish between discharges in the test object and external disturbances, such as background noise. It sometimes makes it possible to determine the type of discharges. Non-electrical detection methods ( 3.5 ) are often ujeful for locating corona on the high-voltage leacl$ or elsewhere in the test area. They may also give independent confirmation of partial discharges in the test object. 7.3 Reduction of Disturbances 7.3.1 General - Reduction of disturbances can be achieved by suitably grounding all conlucting structures in the vicinity of the tests and by The best filtering the power supplies for the test and measuring circuits. reduction is achievable by testing in a shielded room where all electrical Further connection's into the room are marle through effective filters. reduction of disturbances may be achieved by the methods described in 7.3.2and7.3.3. 7.3.2 Balanced Circuits - The use of a balanced circuit, Fig. lC, often enables the observer to dintinguijh bntween discharges in the test object and discharges in other parts of the test circuit, or background noise, and also to compensate for the latter. 7.3.3 Electronic Processing and R:coyerin,o of S!`glals -- Generally ancl especially during inAustria conditions, the sensitivity is limited by the 21 Various electronic presence of disturbances. may be used individually or in combination in partial discharge signal from the disturbances. with special care. Some of these methods are methods do exist, which order to separate the true They should only be used described below: 4 Time window mefhod The instrument may be provided with a gate which may be opened and closed at preselected moments, thus either passing the signal or blocking it. If the disturbances occur during regular intervals the gate may be closed during these intervals. In tests with alternating voltage, the true discharge signal may occur only at regularly repeated intervals during the cycles of the test voltage. This may be used to open the gate only at these intervals. The time window method is particularly useful for tests with direct voltage where the test voltage is obtained by rectifying an alternating voltage. b) Polarity discrimination method Signals originating from the test object may be distinguished from disturbances originating from outside the test cilcuit by comparing the polarity of the pulses across measuring impedances such as zrn and &I, in Fig. 1C. A logic system performs the comparison and operates the gate of the instrument described in (a) above, for the pulses of the correct polarity and consequently only those pulses which originate from the test object are recorded. 4 Pulse averaging Many disturbances in an industrial environment are random whereas true discharges recur at approximately the same time in each cycle of applied voltage. It is therefore possible to greatly reduce the relative level of randomly occurring disturbances by using modern signal-averaging techniques. 4 Frequency selection Broadcast radio interference is limited to discrete bands but will still affect broadband discharge detectors if the transmission frequency falls within the frequency band of the instrument. To reduce this type of interference the gain of the instrument amplifier may be reduced by bandstop filters tuned to the frequencies where the disturbances occur. Alternatively narrow-band instruments may be used which are tuned to a frequency at which the interference level is negligible. 22 X3:6269-1982 7.4 Disturbance Levels 7.4.1 No definite values for the magnitudes of disturbances may be given, but as a general guide, dkturbances equivalent to individual discharges of some hundreds of picocoulombs may be encountered in unscreened industrial testing areas, especially in the case of test circuits of large physical dimensions. By the use of balanced test circuits, such disturbances may be considerably reduced. Tn shielded test rooms, with effective connecting of all conducting structures to the screen and with adequate precautions to supress disturbances from the power supply and from other electrical systems, the ultimate limit of measurement is that of the measuring arrangement itself or that given by minor imperfections in the screening, grounding or filtering. For practical applications today, the lowest measurable value is about 1 PC. 8. SPECIAL REQmEMENTS FOR PARTIAL DISCHARGE TESTS WITH DIRECT MEASUREMENTS DURING VOLTAGE 8.1 General 8.1.1 There are several significant differences between partial discharge phenomena during tests with direct voltage and those with alternating voltage, particularly for tests on solid and liquid insulations and combinations of these. For gaseous insulation these differences may be negligible. Some of these differences are summarized as follows: a) The reoetition rate for direct voltage mav be verv low due to the fact that the time interval betgeen iidividual pulses at direct voltage is determined by the electrical time constant of the materials involved, while for alternating voltage it is determined by the frequency of the test voltage. The voltage distribution within the insulation materials will be b) determined by the resistivities when the voltage is constant while it will be essentially determined by the dielectric constants during voltage variations. After a change of the voltage level, either increase or decrease, 4 there will be a charge redistribution process which normally The same applies to a polarity has a fairly long duration. reversal. d) The partial discharge behaviour of a test object may also be considerably influenced by such parameters as ripple on the direct voltage and temperature. 23 IS : 6269 - 1982 With regards to these phenomena further information concerning some of the 2 to 7 are given in the following. 8.2 Qmntities Related to Partial Discharges 8.2.1 In general, quantities such as apparent charge q and repetition However, there is rate n are also applicable to tests with direct voltages. no experience available concerning the use of integrated quantities for such tests. 8.3 Voltages Related to Partial Discharges 8.3.0 Voltage values during partial discharge tests are given by their mean values in the case of direct voltages. NOTE tude. The ripple factor may in some cases influence the partial discharge m egni. 8.3.1 Partial Discharge Inception and Extinction Voltages - The partial discharge inception and extinction voltages have no practical meaning during tests with direct voltages as they are dependent on factors, such as the voltage distributions under variable voltages. Under certain conditions the partial discharges may continue even This is valid particularly for solid and after removal of the test voltage. liquid insulation and combinations of these. 8.3.2 Partial Discharge Test Voltage - The partial discharge test voltage is cefined similarly to that for alternating voltage. Usually, only discharge magnitude above a certain repetition rate are considered; however, single high magnitude pulses occurring infrequently may be of importance. 8.4 Test Circuits and Measuring Instruments 8.4.1 In general, test circuits and measuring instruments used during tests with alternating voltages may also be used with direct voltages. However, it is recommended that pulse counting devices be used as a complement. When the pulse repetition rate, n, is low, counting devices, which display the number of discharges indifferent, selectable magnitude ranges over each time interval are useful. 8.5 Tests 8.5.1 Requirements for the Test Voltage - For partial discharge tests with direct voltages, the test voltages and the rate of rise shall comply with the requirements of IS : 2071 ( Part II )- 1974,) if not otherwise specified in the relevant Indian standard. *Methods of high voltage testing: Part II Test procedures(jrd mision ). 24 I6 r.6209 --MS2 6.5.2 Choirs of Test `Procedure - The procedures described for alternating voltage to determine the inception and extinction voltages are generally not applicable for tests with direct voltage as the stress on the dielectric during voltage rise and decrease is different from the one with constant voltage. There is no generally accepted method for the determination of the P.D. magnitude during tests with direct voltage. Whatever method is used, it is important to note that the partial discharge magnitude at the beginning of the voltage application may be different from its magnitude after a considerable time at the same test voltage. 6.6 Dieturbanc+s 8.6.1 The information given in 7 is also applicable for tests with direct voltages. However, in this case a particular type of regularly repeated disturbance may occur which is related to the transition of current in the rectifier elements of the direct voltage source. APPENDIX ( Clause 3.2.2 ) TEST CIRCUITS A-O. GENERAL A A-O.1 Test cncuits for the measurement of partial discharges either have the measuring impedance connected in series between the test object and ground or the measuring impedance is connected across the test object With the series connection, by means of a suitable coupling capacitor. some of the partial discharge currents may bypass the measuring impedance if the test object is not encased in such a manner as to ensure that all of the currents are collected and forced to flow through the measuring impedance. A-l. CIRCUITS A-1.0 There are three basic circuits from which all other test circuits for These the detection and measurement of partial discharges are derived. three circuits, which are shown in Fig. 1 (a), 1 (b) and 1 (c) are briefly described below. A-l.1 Figure la - The measuring impedance in this circuit is placed at the earth side of the coupling capacitor. This arrangement has the advantage of being suitable for testing objects having one earthed 25 X5:6209- 1982 terminal, the test object being connected directly between the high The impedance < between the test object and voltage source an-l earth. the high voltage source serves to attenuate disturbances from the high voltage source. It also increaser sensitivity in the measurements by providing blocking of the pulses from the test object which would otherwise be bypassed through the source impedance. A-1.2 Figure lb - In this circuit. the measuring impedance is placed at the earth side of the test object. The low-voltage side of the test object should therefore be isolated from earth. - A circuit is sometimes used which is similar to that shown in Fig. lb, but NOTE This arrangein which, the function of Ck is performed by the stray capacitances. ment may be suitable if the capacitance of the test object is-small compared with Be It may also be satisfactory if the terminal capacitance of stray capacitance to earth. the testing transformer is at least of the same order as C,, provided that 2 is omitted. A-l.3 Figure lc - The arrangement shown comprises a balanced circuit in which the instrument is connected between the impedances 5m and SW. The low voltage side of the test object and the coupling capacitance should both be isolated from earth. The capacitances of the.parts connected to zrn and zrnl' need not be equal but should preferably be of the same order, anl for the best results their dielectric loss factors particularly in relation to their frequency dependence should be similar. The circuit has the merit of partially rejecting external To adjust this rejection an artificial discharge source may disturbances. be coupled between the high-voltage terminal and earth. The impedance zrn or xrnl is then adjusted until maximum reduction of the instrument Reduction ratios from 3 reading is obtained. ( for totally unequal test objects .) to 1 000 or even higher ( for identical well screened test objects ) are possible. APPENDIX B ( Clawes 2.2.4 and 4.2.2 ) INTEGRATED QUANTITIES B-l. The inteqrated quantities are related to the apparent charge q and the repetition rate n as follows when T is a reference time interval: a) Average discharge current I 26 ISr6209-1982 The average discharge current is expressed in coulombs per second or ampere. In some cases the time interval is one cycle and the quantity is referred to as total apparent charge per cycle. b) Quadraiic rak D DThe quadratic + ( q1' + qg + . . . qma ) rate is expressed in ( coulomb3 )" per second. Calibration of instruments measuring I or D in the complete test arrangement is made in a similar way to that described in 4.3 for the measurement of q. The repetition rate of the generator should be lower This requirement is than the bandwidth of the measuring instrument. generally met if the repetition rate corresponds to a pulse interval greater than the resolution time, which, however, is not necessary for measuring the quardratic rate. In addition, the pulse repetition rate, n, should be known. If the pulses are derived from a rectangular voltage generator .of fundamental frequency, fg, and if both positive and negative current pulses are used, the repetition rate n will be equal to 2 fg. Under these ,conditions the instrument reading corresponds to an average discharge current: I = 2fg uo Co and a quadratic c) Dbchargc power P rate D = 2fe ( Uo Co )' where PI, ~3, pm are absolute instaneous values of the test voltage at the instants of discharges ql, qs, ._., qm, The average discharge power is expressed in wattr. APPENDIX MEASUREMENTS ,C-0. GENERAL ON CABLES AND WITH WINDINGS C ON TEST OBJECTS ( Clauses 2.2.2, 3.2.1, 4.3.3 and 5.5.1 ) C-O.1 In principle, any of the test circuits described in Appendix A can be used for test objects with windings and for cables - that is for test 27 X6:6269-I982 objects with distributed capacitive and inductive elements. For some of these test objects, the test voltage may be induced; for example, the highvoltage winding of a transformer may be excited from the low-voltage winding, Fig. 3. C-l. POINTS FOR CONSIDERATION C-1.0 A detailed treatment of partial discharge measurements on objects with distributed elements is beyond the scope of this standard. The following points, however, are of special importance. C-l.1 Attenuation Phenomena - Due to attenuation within windings. or along cables, the magnitude which is recorded at a terminal of the test object may differ in magnitude from that at the point where it originates. C-l.2 Resonance Phenomena, Reflections -The magnitude recorded at a terminal of a winding or cable under test may be modified by resonance phenomena or by reflections at the terminals. This is especially important ifthe instrument used has a narrow-band frequency response. Reflection phenomena ( for example, in cables ) can be taken into account using special calibration techniques such as double pulse generators. C-l.3 Impedance Characteristics - A test object with windings does not behave as a simple capacitance C,, but often has the characteristics of a surge impedance, generally with some parallel lumped capacitance. C-l.4 Location of Discharges - Various methods can be used to locate partial discharges in test objects with windings or in cables. Some of these ,methods are based on simultaneous measurements at two or more terminals of the test object. Non-electrical methods may also be applicable. APPENDIX ( Clause 3.4.7 ) THE USE OF RADIO MEASUREMENT INTERFERENCE OF PARTIAL D METERS FOR THE DISCHARGES D-l. Requirements for instruments in common use for radio interference Their response is measurements are given in various specifications. generally determined by tuned bandpass filters, having a specified narrow bandwidth and variable midband frequency, and by a quasipeak measuring circuit with specified charging time constant ~1 and a 28 ISr6!26!3-l9S2 discharging instrument, time constant 3. critically damped The indicating meter is a moving coil and having a mechanical time constant cs. D-2. The characteristic of the instrument makes it respond basically to the charge of an input current pulse. Due to the quasi-peak measuring circuit of this instrument, an impulse having the same charge but a higher repetition rate results in a higher reading on the instrument. The meter reading Ur, depends on both the partial discharge q and the repetition rate n. q can be determined from: !? =A. Ur pulses and in a given circuit the following magnitude For short, regularly repeated approximation is applicable: where f(n) A is a non-linear is the instrument function of n f bandwidth measuring depends impedance on the test pro- & is the value of a purely resistive B is a coefficient the value circuit and the test object. of which A is a coefficient which is established during calibration ,viding that the criteria described below are fulfilled. The reading can thus be considered to be approximately proportional to the magnitude q and to the instrument bandwidth. It may not in practice be proportional to & if this has stray capacitance or inductance. The `Tactorf(n) is not strictly applicable if the discharge pulses are irregularly distributed in time. One such instrument, designed as a quasi-peak voltmeter, is described in This specifies a bandwidth A f at 6 dB of IS : 10052 ( Part I )-1982*. 9 kHz and time constants of ~1 = 1 ms, TS= 1CO ms and ~8 = 160 ms. For the measurement of radio interference, the instrument is calibrated using a sinusoidal voltage at the frequency to which the instrument is tuned and the interference voltage is conventionally expressed as the rms value of Short and constant pulses of O-158 pVs an equivalent sinusoidal voltage. applied to the instrument with a regular repetition rate of 100 per second *Electromagnetic interference measuring a paratus and measuring methods : Part I Measuring apparatus of the frequency range 1B KHz to 1 GHz. 29 IS:6209 -1982 should give the same reading as a sine wave input of 1 000 PV rms at the tuned frequency. The variation of the reading with repetition rate n for this instrument is shown in Fig. 5. The above-mentioned IS gives specifications for the use of this instrument for measurement of the radio noise voltage generated by high-voltage equipment. Two test circuits are described therein which agree essentially with those of Fig. la and lb, and may also be used for the measurement of partial discharges with certain precautions. +J. 5 WITH REPETITION RATE n FOR CONSTANTPULSES VARIATION OF RADIO INTERFERENCEMETER READING It should ,be noted that the curve of Fig. 5 applies to regularly repeated pulses only. Consequently, if a radio interference meter is to be used for partial discharge measurements, it should be calibrated and checked It is recommended that this be in the actual circuit according to 4.3. done by the application of regularly repeated pulses qo having a repetition rate equal to approximately twice the frequency of the test voltage. This will enable the instrument to be used for the measurement of partial discharge magnitude during an actual test near the inception voltage where the number of pulses per cycle is small. The partial discharge magnitude under these conditions is then approximately equal to q. multiplied by the ratio of the instrument reading during the test to thqt This relationship also applies over a limited during the calibration. range of pulse repetition rates where the variation of readings due to the factorf(n) is small. Whenever measurements made with a radio interference meter are quoted, both the value in microvolts and an estimated equivalent discharge magnitude should preferably be stated. 30 BUREAU Headquarters: OF INDIAN STANDARDS Manak Bhavan, 9 Bahadur Shah tafar Marg. NEW DELHI 110002 Telephones: 331 01 31, 331 13 75 Regional Offices: Telegrams: Manaksanstha ( Common to all Offices) Telephone Central : Manak Bhavan, 9 Bahadur Shah Zafar Marg, NEW DELHI 110002 .Eastern : l/l 4 C. I. T. Scheme VII M, V. I. P. Road. Maniktola, CALCUTTA 700054 N&hern : SC0 445-446, Sector 35-C, CHANDIGARH 160036 I 331 01 31 331 1375 36 24 99 21843 I 3 16 41 41 24 42 Southern : C. I. T. 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